The present invention relates to the automatic control of performance of spark ignited internal combustion engines and more particularly to a method and apparatus for retarding the spark of the engine when knocking occurs and to return the spark to its normal condition when knock ceases.
In the field of internal combustion engines, detonation, or knock, is an undesirable event. In the past it has been avoided by limiting compression ratio to 10.5:1 or less, and by providing gasoline with sufficient octane, up to 100 RON, to satisfy these engines. The unleaded gasoline required in 1975 and later vehicles is generally available only at lower octane, approximately 91 RON. This octane level will satisfy cars with compression ratios no higher than approximately 8:1. However, lowering compression ratio to 8:1 from the historic average of slightly over 9:1 involves a penalty of about 7% fuel economy.
Some of this lost fuel economy can be regained when a higher compression ratio is used along with other means to avoid knock at those conditions in which the engine is most knockprone, for example, at full throttle acceleration. Two known methods of avoiding knock are spark retard and use of exhaust gas recirculation. Both techniques lower the peak temperatures of combustion and therefore the tendency to knock. Accordingly both are suited to use with higher compression ratio engines.
Recent developments in electronic control over the ignition system for internal combustion engines has been the basis for utilizing spark retard as the means to eliminate or regulate engine knock. Knock can be detected in an engine by transducers, e.g. a strain gauge or accelerometer, placed on the engine, for example, on the main bearing cap, cylinder heads, intake manifold, or block. Knock causes a significantly larger than normal force which, as is known, is translated into a millivolt signal by the transducer. Knock is characterized by increased engine vibration generally in frequency ranges of 4000 to 6000 Hz and 9000-11,000 Hz, and transducers are selected to be responsive in those ranges. Knock might occur once in each engine cylinder for each firing of that cylinder. The amplitude of a knock pulse, in terms of the millivolt signals produced by the transducer, can be as much as twenty times greater than the signal of background noise produced by normal engine operation without knocking. Background noise varies in a generally direct relationship with engine space and load and knock pulses are characterized by as many as twelve cycles in the above frequency ranges, whose amplitudes are higher than background noise at that speed.
Many electronic spark control circuits have been developed in the past either for regulation of engine performance or for application in rating of gasoline. For example, U.S. Pat. No. 3,822,583 by Keller, et al. discloses an apparatus to measure knock ratings of gasoline and to indicate the octane requirements of an engine. This apparatus uses a transducer to pick up engine background noise and knock; these signals then are filtered in the preferred knock frequency range and amplified. The engine signals, including knock, are compared with the average background noise level and any positive difference produced by knock is integrated to provide a feedback signal to a servo device operating the spark control.
U.S. Pat. No. 4,002,155 by Harned and Herrick discloses a spark timing control using an engine mounted accelerometer to monitor knock. When knock pulses exceed a predetermined number, the spark is retarded. Spark is advanced when less than the predetermined number of knocks occur.
In these systems the knock signal level contributes to the comparison signal level, thus degenerating the comparator output and sensitivity to knock.
A spark control method and apparatus are needed which allow flexibility in control of parameters affecting performance such as degree of retard, sensitivity to knock, time of retard, speed of spark advance.
Additionally a method and apparatus for spark control are needed which provide a control signal output representative of knock rate and intensity regardless of engine background noise variations. Also a method and apparatus for spark control are needed which prevent control hunting when periods of knocking are separated by short intervals.